A uterine contraction is a temporary process during which the muscles of the uterus are shortened and the space between muscle cells decreases. These structural changes in the muscle cause an increase in uterine cavity pressure to allow pushing the fetus downward in a lower position towards delivery. During a uterine contraction, the structure of myometrial cells (i.e., cells of the uterus) changes and the uterine wall becomes thicker. FIG. 1A shows an illustration of a relaxed uterus, in which the muscular wall of the uterus is relaxed. FIG. 1B shows an illustration of a contracted uterus, in which the muscular wall of the uterus contracts and pushes the fetus against the cervix.
Uterine contractions are monitored to evaluate the progress of labor. Typically, the progress of labor is monitored through the use of two sensors: a tocodynamometer, which is a strain gauge-based sensor positioned on the abdomen of an expectant mother, and an ultrasound transducer, which is also positioned on the abdomen. The signals of the tocodynamometer are used to provide a tocograph (“TOCO”), which is analyzed to identify uterine contractions, while the signals of the ultrasound transducer are used to detect fetal heart rate, maternal heart rate, and fetal movement. However, these sensors can be uncomfortable to wear, and can produce unreliable data when worn by obese expectant mothers.